Header assembly for optoelectronic devices

ABSTRACT

A header assembly is provided that includes a base having a device side and a connector side. The header assembly further includes a platform attached to the base and positioned in a predetermined orientation with respect to the base. The device side of the base cooperates with a cap to define a hermetic chamber wherein one or more optoelectronic components, such as optical transmitters and optical receivers, are disposed. The platform includes an inside portion proximate the device side of the base and an outside portion proximate the connector side of the base, and the platform further includes at least one conductive pathway extending substantially through the platform so as to facilitate electrical communication between components disposed on the device side of the base, and circuits, devices and systems disposed on the connector side of the base.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 10/231,395, filed Aug. 29, 2002, entitled “Header AssemblyHaving Integrated Cooling Device” which is a continuation-in-part of thefollowing applications: U.S. patent application Ser. No. 10/077,067,filed Feb. 14, 2002, entitled “Ceramic Header Assembly” (now U.S. Pat.No. 6,586,678); and U.S. patent application Ser. No. 10/101,260, filedMar. 18, 2002, (claiming priority to U.S. Provisional Patent ApplicationSerial No. 60/317,835, filed Sep. 6, 2001), entitled “Compact LaserPackage with Integrated Temperature Control.” All of the aforementionedpatents and patent applications are incorporated herein in theirrespective entireties by this reference.

BACKGROUND

[0002] 1. Technological Field

[0003] This invention is generally concerned with the field ofopto-electronic systems and devices. More specifically, embodiments ofthe present invention relate to a header assembly for use in variousoptoelectronic devices.

[0004] 2. Related Technology

[0005] Transistor headers, or transistor outlines (“TO”), are widelyused in the field of opto-electronics, and may be employed in a varietyof applications. As an example, transistor headers are sometimes used toprotect sensitive electrical devices, and to electrically connect suchdevices to components such as printed circuit boards (“PCB”).

[0006] With respect to their construction, transistor headers oftenconsist of a cylindrical metallic base with a number of conductive leadsextending completely through, and generally perpendicular to, the base.A glass hermetic seal between the conductive leads and the base providesmechanical and environmental protection for the components contained inthe TO package, and electrically isolates the conductive leads from themetallic material of the base. Typically, one of the conductive leads isa ground lead that may be electrically connected directly to the base.

[0007] Various types of devices are mounted on one side of the base ofthe header and connected to the leads. Generally, a cap is used toenclose the side of the base where such devices are mounted, so as toform a chamber that helps prevent contamination or damage to thosedevice(s). The specific characteristics of the cap and header generallyrelate to the application and the particular device being mounted on thebase of the header. By way of example, in applications where an opticaldevice is required to be mounted on the header, the cap is at leastpartially transparent so to allow an optical signal generated by theoptical device to be transmitted from the TO package.

[0008] Although transistor headers have proven useful, typicalconfigurations nevertheless pose a variety of unresolved problems. Someof such problems relate specifically to the physical configuration anddisposition of the conductive leads in the header base. As an example,various factors conspire to compromise the ability to precisely controlthe electrical impedance of the glass/metal feedthru, that is, thephysical bond between the conductive lead and the header base material.One such factor is the fact that there is a relatively limited number ofavailable choices with respect to the diameter of the conductive leadsthat are to be employed. Further, the range of dielectric values of thesealing glass typically employed in these configurations is relativelysmall. And, with respect to the disposition of the conductive leads, ithas proven relatively difficult in some instances to control theposition of the lead with respect to the through hole in the headerbase.

[0009] Yet other problems in the field concern those complex electricaland electronic devices that require many isolated electrical connectionsin order to function properly. Typically, attributes such as the sizeand shape of such devices and their subcomponents are sharplyconstrained by various form factors, other dimensional requirements, andspace limitations within the device. Consistent with such form factors,dimensional requirements, and space limitations, the diameter of atypical header is relatively small and, correspondingly, the number ofleads that can be disposed in the base of the header, sometimes referredto as the input/output (“I/O”) density, is relatively small as well.

[0010] Thus, while the diameter of the header base, and thus the I/Odensity, may be increased to the extent necessary to ensure conformancewith the electrical connection requirements of the associated device,the increase in base diameter is sharply limited, if not foreclosedcompletely, by the form factors, dimensional requirements, and spacelimitations associated with the device wherein the transistor header isto be employed.

[0011] A related problem with many transistor headers concerns theimplications that a relatively small number of conductive leads has withrespect to the overall performance of the device wherein the transistorheader is used. Specifically, devices such as semiconductor lasersoperate more efficiently if their driving impedance is balanced with theimpedance at the terminals. Impedance matching is often accomplishedthrough the use of additional electrical components such as resistors,capacitors and transmission lines such as microstrips or striplines.However, such components cannot be employed unless a sufficient numberof conductive leads are available in the transistor header. Thus, thelimited number of conductive leads present in typical transistor headershas a direct negative effect on the performance of the semiconductorlaser or other device.

[0012] In connection with the foregoing, another aspect of manytransistor headers that forecloses the use of, for example, componentsrequired for impedance matching, is the relatively limited physicalspace available on standard headers. In particular, the relatively smallamount of space on the base of the header imposes a practical limit onthe number of components that may be mounted there. In order to overcomethat limit, some or all of any additional components desired to be usedmust instead be mounted on the printed circuit board, some distance awayfrom the laser or other device contained within the transistor header.Such arrangements are not without their shortcomings however, as theperformance of active devices in the transistor header, such as lasersand integrated circuits, depends to some extent on the physicalproximity of related electrical and electronic components.

[0013] The problems associated with various typical transistor headersare not, however, limited solely to geometric considerations andlimitations. Yet other problems relate to the heat generated bycomponents within, and external to, the transistor header. Specifically,transistor headers and their associated subcomponents may generatesignificant heat during operation. It is generally necessary to reliablyand efficiently remove such heat in order to optimize performance andextend the useful life of the device.

[0014] However, transistor headers are often composed primarily ofmaterials, Kovar® for example, that are not particularly good thermalconductors. Such poor thermal conductivity does little to alleviate heatbuildup problems in the transistor header components and may, in fact,exacerbate such problems. Various cooling techniques and devices havebeen employed in an effort to address this problem, but with onlylimited success.

[0015] By way of example, solid state heat exchangers may be used toremove some heat from transistor header components. However, theeffectiveness of such heat exchangers is typically compromised by thefact that, due to variables such as their configuration and/or physicallocation relative to the primary component(s) to be cooled, such heatexchangers frequently experience a passive heat load that is imposed bysecondary components or transistor header structures not generallyintended to be cooled by the heat exchanger. The imposition on the heatexchanger of such passive heat loads thus decreases the amount of heatthe heat exchanger can effectively remove from the primary componentthat is desired to be cooled, thereby compromising the performance ofthe primary component.

[0016] As suggested above, the physical location of the heat exchangeror other cooling device has various implications with respect to theperformance of the components employed present in the transistor header.On particular problem that arises in the context of thermoelectriccooler (“TEC”) type heat exchangers relates to the fact that TECs havehot and cold junctions. The cold junction, in particular, can causecondensation if the TEC is located in a sufficiently humid environment.Such condensation may materially impair the operation of components inthe transistor header, and elsewhere.

[0017] Another concern with respect to heat exchangers is that thedimensions of typical transistor headers are, as noted earlier,constrained by various factors. Thus, while the passive heat load placedon a heat exchanger could be at least partly offset through the use of arelatively larger heat exchanger, the diametric and other constraintsimposed on transistor headers by form factor requirements and otherconsiderations place practical limits on the maximum size of the heatexchanger.

[0018] Finally, even if a relatively large heat exchanger could beemployed in an attempt to offset the effects of passive heat loads,large heat exchangers present problems in cases where the heatexchanger, such as a TEC, is used to modify the performance oftransistor header components such as lasers. For example, by virtue oftheir relatively large size, such heat exchangers are not well suited toimplementing the rapid changes in laser performance that are required inmany applications because such large heat exchangers heat up and cooldown relatively slowly. Moreover, the performance of the laser or othercomponent may be further compromised if the heat exchanger is locatedrelatively far away from the laser because the rate at which heat can betransferred with respect to the laser or other component is at leastpartially a function of the distance between the component and the heatexchanger.

[0019] In view of the foregoing discussion, what is needed is atransistor header having features directed to addressing the foregoingexemplary concerns, as well as other concerns not specificallyenumerated herein. An exemplary transistor header should implement arelatively high I/O density without increasing the relative diameter ofthe header. Moreover, the exemplary transistor header should beconfigured to precisely control the electrical impedance and permitlocation of various components in relatively close proximity to theactive components, such as a laser, within the header without violatingapplicable form factors or other geometric and dimensional standards.Finally, the exemplary transistor header should include featuresdirected to facilitating a relative improvement in heat managementcapability within the transistor header.

BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION

[0020] In general, embodiments of the invention are concerned with atransistor header including various features directed to enhancing thereliability and performance of various electronic devices, such aslasers, included in the transistor header.

[0021] In one exemplary embodiment, a transistor header is provided thatincludes a substantially cylindrical metallic base as well as a platformdisposed in a substantially perpendicular orientation with respect tothe base and extending through both sides of the base. The platform isconstructed from an insulating material such as a ceramic. The platformis hermetically sealed to the base, and flat surfaces defined by theplatform on either side of the base are configured to receive multipleelectrical components. Moreover, the platform includes a plurality ofconductive pathway(s) extending between the ends of the platform so thatcomponents on opposite sides of the base may be electrically connectedwith each other. On one end of the platform, a connector is providedthat is in electrical communication with some or all of such conductivepathways.

[0022] In this exemplary embodiment, a laser is disposed on top of aTEC, which, in turn, is mounted to the platform. A cup having atransparent portion is situated on the base cooperates with the platformand the base to define a hermetic chamber enclosing the laser and theTEC. Power is supplied to the TEC by way of a laser control system thatcommunicates both with a light intensity measuring device opticallycoupled to the laser and with a temperature sensing device thermallycoupled to the laser.

[0023] In operation, power is supplied to the laser by way of theconnector on the platform and the laser emits light through thetransparent portion of the cup. The light intensity measuring device andthe temperature sensing device provide data on the light intensity ofthe laser as a function of laser temperature and transmit the data to acontrol circuit which adjusts the power applied to the TEC, therebyraising or lowering the temperature of the laser as necessary to meetthe laser performance requirements.

[0024] These and other, aspects of embodiments of the present inventionwill become more fully apparent from the following description andappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order that the manner in which the above-recited and otheradvantages and features of the invention are obtained, a more particulardescription of the invention briefly described above will be rendered byreference to specific embodiments thereof which are illustrated in theappended drawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be consideredlimiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

[0026]FIG. 1A is a perspective view illustrating various aspects of thedevice side of an exemplary embodiment of a header assembly;

[0027]FIG. 1B is a perspective view illustrating various aspects of theconnector side of an exemplary embodiment of a header assembly;

[0028]FIG. 2A is a perspective view illustrating various aspects of thedevice side of an alternative embodiment of a header assembly;

[0029]FIG. 2B is a perspective view illustrating various aspects of theconnector side of an alternative embodiment of a header assembly;

[0030]FIG. 3A is a perspective view illustrating various aspects of thedevice side of another alternative embodiment of a header assembly;

[0031]FIG. 3B is a perspective view illustrating various aspects of theconnector side of another alternative embodiment of a header assembly;

[0032]FIG. 4A is a top perspective view of an exemplary embodiment of aheader including active devices mounted on a TEC disposed within ahermetic chamber;

[0033]FIG. 4B is a bottom perspective view of the exemplary embodimentillustrated in FIG. 4A;

[0034]FIG. 4C is a cross-section view illustrating various aspects ofthe exemplary embodiment presented in FIGS. 4A and 4B;

[0035]FIG. 4D is a cross-section view taken along line 4D-4D of FIG. 4Cand illustrates various aspects of an exemplary arrangement of a TEC ina header assembly;

[0036]FIG. 4E is a side view illustrating aspects of an exemplaryelectrical connection scheme for the header assembly and a printedcircuit board;

[0037]FIG. 4F illustrates various aspects of an alternative platform/TECconfiguration where the TEC is located outside the hermetic chamber; and

[0038]FIG. 5 is a schematic diagram illustrating various aspects of anexemplary embodiments of a laser control system.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0039] Reference will now be made to figures wherein like structureswill be provided with like reference designations. It is to beunderstood that the drawings are diagrammatic and schematicrepresentations of various embodiments of the claimed invention, and arenot to be construed as limiting the scope of the present invention inany way, nor are the drawings necessarily drawn to scale.

[0040] Reference is first made to FIGS. 1A and 1B together, whichillustrate perspective views of one presently preferred embodiment of aheader assembly, designated generally at 200. In the illustratedexample, the header assembly 200 includes a substantially cylindricalmetallic base 10. The base 10 includes two flanges 90 used to controlangular or rotational alignment of the header 200 to a receptacle (notshown) on a higher level opto-mechanical assembly. The base can beformed of Alloy 42, which is an iron nickel alloy, as well ascold-rolled steel, or Vacon VCF-25 Alloy. The base 10 also includes aceramic platform 70 extending perpendicularly through the base as shown.The ceramic platform is hermetically sealed to the base to providemechanical and environmental protection for the components contained inthe TO package.

[0041] The hermetic seal between the base 10 and the platform 70 iscreated by electrically insulating glass-to-metal seals. Alternatively,the platform 70 may incorporate two additional ceramic outer layers toelectrically isolate the outermost conductors. In this second case, ametal braze or solder can be used to hermetically seal the platform 70to the metal base. This solution overcomes the principal shortcomings ofglasses, namely their low strength, brittleness, and low thermalconductivity.

[0042] The platform 70 is structured to house multiple electricalcomponents 50 and 100, and active devices 60 on either side of the base.In the illustrated embodiment, the active device 60 comprises asemiconductor laser, and the components 50 and 100 are resistors,capacitors, and inductors that are used to balance the driving impedanceof the laser with the component impedance. As it is important for asemiconductor laser to be precisely positioned perpendicularly to thebase 10, platform 70 is, therefore, precisely positioned perpendicularlywith respect to the base 10.

[0043] Where active device 60 comprises a semiconductor laser, a smalldeviation in the position of active device 60, in relation to base 10can cause a large deviation in the direction of the emitted laser beam.Accurate perpendicularity between the platform and the base can beachieved by incorporating a vertical pedestal feature in the basematerial, as shown on FIG. 1A. The vertical pedestal houses thephotodiode 30 in the embodiment shown in FIG. 1A. Such feature can bemachined, stamped, or metal injection molded directly with the base thusproviding a stable and geometrically accurate surface for mating withthe platform.

[0044] The platform 70 further includes multiple electrically isolatedconductive pathways 110 extending throughout the platform 70 andconsequently through the base 10. The conductive pathways 110 providethe electrical connections necessary between electrical devices orcomponents located throughout the platform 70. The conductive pathways110 form a connector on that side of the base that does not include thesemiconductor laser 60, also referred to herein as the “connector side”of the base. Note in connection with the foregoing that the side of thebase where the active device 60 is located may in some instances bereferred to herein as the “device side” of the base.

[0045] The connector formed by the conductive pathways 110 is used toelectrically connect the header assembly 200 to a second electricalsubassembly, such as a printed circuit board, either directly (forexample, by solder connection) or indirectly by an intermediary devicesuch as a flexible printed circuit. The semiconductor laser 60 iselectrically connected to the electrical components 50 and 100 via theconductive pathways 110. In one embodiment, the platform 70 is itself aprinted circuit board having conductive pathways 110 formed therein.

[0046] The use of advanced ceramic materials, examples of which includealuminum nitride and beryllia, allows the header assembly 200 to achievesubstantially lower thermal resistances between the devices inside thepackage and the outside world where heat is ultimately transferred. Asdiscussed in further detail below in the context of an alternativeembodiment of the invention, a cooling device, such as a thermoelectriccooler (“TEC”), a heat pipe or a metal heat spreader, can be mounteddirectly on the platform, thereby providing for a very short thermalpath between the temperature sensitive devices on the platform and aheat sink located outside the header assembly.

[0047] As is further shown in FIGS. 1A and 1B, the header assembly 200can additionally include two conductive leads 40 extending through andout both sides of the base 10. The conductive leads 40 are hermeticallysealed to the base 10 to provide mechanical and environmental protectionfor the components contained in the TO package between the conductiveleads 40 and the base 10. The hermetic seal between the conductive leads40 and the base 10 is created, for example, by glass or other comparablehermetic insulating materials that are known in the art. The conductiveleads 40 can also be used to electrically connect devices and/orcomponents located on opposite sides of the base.

[0048] In the illustrated embodiment at least, the conductive leads 40extend out from the side of the base 10 that does not contain thesemiconductor laser 60, in a manner that allows for the electricalconnection of the header assembly 200 with a specific header receptaclelocated on, for example, a printed circuit board. It is important tonote that conductive pathways 110 and conductive leads 40 perform thesame function and that the number of potential conductive pathways 110is far greater than the potential number of conductive leads 40.Therefore, alternative embodiments can incorporate even more conductivepathways 110 than shown in the illustrated embodiment.

[0049] The platform 70 further includes steps and recessed areas thatpermit mounting devices with various thicknesses flush with the metalpads on the ceramic. This allows the use of the shortest electricalinterconnects, wire bonds for example, having improved electricalperformance and characteristics.

[0050] The photodiode 30 is used to detect the signal strength of thesemiconductor laser 60 and relay this information back to controlcircuitry (see FIG. 5) of the semiconductor laser 60. In the illustratedembodiment, the photodiode can be directly connected to the conductiveleads 40. Alternatively, the photodiode can be mounted directly onto thesame platform as the laser, in a recessed position with respect to thelight emitting area. This recessed position allows the photodiode tocapture a fraction of the light emitted by the laser, thus allowing thephotodiode to perform the same monitoring function. In yet anotherconfiguration, as shown in FIG. 4C, a monitor photodiode 1004 with anangled facet can be mounted in a plane behind the laser diode. Theangled facet deflects the light emitted from the back-facet of the laserupwards toward the sensitive area of the detector.

[0051] The configurations of the monitoring photodiode discussed in theprevious paragraph allow for eliminating the need of conductive leads40, and lends themselves to simplified electrical connections, such aswire bonds, to the conductive pathways 110 of the platform 70. In analternative embodiment, the photodiode light gathering can be increasedby positioning an optical element on the base for focusing orredirecting light, such as a mirror, or by directly shaping and/orcoating the base metal to focus additional light onto the photodiode

[0052] As is further shown in FIG. 1A, the base 10 includes a protrudingportion 45 that is configured to releasably position or locate a cap(not shown) over one side of the base 10. A cap can be placed over theside of the base 10 containing the semiconductor laser 60 for thepurpose of protecting the semiconductor laser 60 from potentiallydestructive particles. A transparent cap is preferable for theillustrated embodiment so as to allow the laser light to escape theregion between the cap and the base 10.

[0053] Reference is next made to FIGS. 2A and 2B, which illustrateperspective views of an alternative embodiment of a header assembly,designated generally at 300. This alternative embodiment shows anoptical receiver 360 mounted horizontally on the platform 370perpendicularly bisecting the base 310 of the header assembly 300. Theoptical receiver can be a photodetector or any other device capable ofreceiving optical signals. The optical receiver 360 is mounted flat onthe platform 370 and detects light signals through the side facing awayfrom the base 310. This type of optical receiver is sometimes referredto as an “edge detecting” detector. The base 310 and platform 370 aredescribed in more detail with reference to FIGS. 1A and 1B. The platform370 contains electrical components 350, 400 on either side of the basefor operating the optical receiver 360. The platform 370 also includesconductive pathways 410 for electrically connecting devices orcomponents on either side of the base 310. This embodiment of a headerassembly does not contain conductive leads and therefore all electricalconnections are made via the conductive pathways 410.

[0054] Reference is next made to FIGS. 3A and 3B, which illustrateperspective views of yet another alternative embodiment of a headerassembly, designated generally at 500. This alternative embodiment alsoshows an optical receiver 530 mounted vertically on the base 510. Theoptical receiver can be a photodetector or any other device capable ofreceiving optical signals. This is an optical receiver 530 which detectslight signals from the top of the device. The base 510 and platform 570are described in more detail with reference to FIGS. 1A and 1B. Theplatform 570 contains electrical components 550, 600 on either side ofthe base for operating the optical receiver 530. The platform 570 alsoincludes conductive pathways 510 for electrically connecting devices orcomponents on either side of the base 510. This embodiment of a headerassembly does not contain conductive leads and therefore all electricalconnections are made via the conductive pathways 410.

[0055] Directing attention now to FIGS. 4A through 4D, various aspectsof an alternative embodiment of a header assembly, generally designatedat 700, are illustrated. The embodiment of the header assemblyillustrated in FIGS. 4A through 4D is similar in many regards to one ormore of the embodiments of the header assembly illustrated in FIGS. 1Athrough 3B. Accordingly, the discussions of FIGS. 4A through 4D willfocus primarily on certain selected aspects of the header assembly 700illustrated there. Note that in one embodiment of the invention, headerassembly 700 comprises a transistor header. However, header assembly 700is not limited solely to that exemplary embodiment.

[0056] As indicated in FIGS. 4A through 4D, header assembly 700generally includes a base 702 through which a platform 800 passes.Platform 800 may comprise a printed circuit board or, as discussedherein, may comprise other materials and/or configurations as well. Theplatform 800 is configured to receive a cooling device 900 upon whichvarious devices and circuitry are mounted. Note that while it may bereferred to herein as a “cooling” device 900, the cooling device 900may, depending upon its type and the application where it is employed,serve both to heat and/or cool various components and devices. Finally,a cap 704 mounted to, and cooperating with, base 702, serves to define ahermetic chamber 706 which encloses cooling device 900 and the mounteddevices and circuitry.

[0057] As discussed in further detail below, a variety of means may beemployed to perform the functions disclosed herein, of a cooling device.Thus, the embodiments of the cooling device disclosed and discussedherein are but exemplary structures that function as a means fortransferring heat. Accordingly, it should be understood that suchstructural configurations are presented herein solely by way of exampleand should not be construed as limiting the scope of the presentinvention in any way. Rather, any other structure or combination ofstructures effective in implementing the functionality disclosed hereinmay likewise be employed.

[0058] With continuing attention to FIGS. 4A and 4B, and directingattention also to FIGS. 4C and 4D, further details are providedconcerning various aspects of platform 800. In the illustratedembodiment, platform 800 is disposed substantially perpendicularly withrespect to base 702. In particular, base 702 includes a device side 702Aand a connector side 702B, and platform 800 passes completely throughbase 702, so that an inside portion 801A of platform 800 is disposed ondevice side 702A of base 700 and outside portion 801B of platform 800 isdisposed on connector side 702B of base 702. However, this arrangementof platform 800 is exemplary only, and various other arrangements ofplatform 800 may alternatively be employed consistent with therequirements of a particular application.

[0059] In the illustrated embodiment, platform 800 includes a firstfeedthru 802 having a multi-layer construction that includes one or morelayers 804 of conductive pathways 806 (see FIG. 4A). In general,conductive pathways 806 permit electrical communication among thevarious components and devices (removed for clarity) disposed onplatform 800, while also permitting such components and devices toelectrically communicate with other components and devices that are nota part of platform 800. Moreover, conductive pathways 806 cooperate toform a connector 810 situated on the outside portion 801B of platform800, on the connector side 702B of base 700. In general, connector 810facilitates electrical communication between header assembly 700 andother components and devices such as, but not limited to, printedcircuit boards (see FIG. 4E). In one embodiment, connector 810 comprisesan edge connector, but any other form of connector may alternatively beused, consistent with the requirements of a particular application. Asdiscussed in further detail below, first feedthru 802 may includecutouts 811 or other geometric features which permit direct access to,and electrical connection with, one or more conductive pathways 806disposed on an inner layer of first feedthru 802.

[0060] In addition to the first feedthru 802, platform 800 furtherincludes a second feedthru 812 to which the first feedthru 802 isattached. Note that in the exemplary illustrated embodiment, firstfeedthru 810, with the exception of conductive pathways 806, maycomprise a material that is generally resistant to heat conduction, suchas a ceramic with low thermal conductivity, such as alumina for example.Low thermal conductivity ceramics may be more desirable in someinstances than high thermal conductivity ceramics, such as aluminumnitrade or beryllia, due to the relatively lower cost of such lowthermal conductivity ceramics, as well as the ease with which such lowthermal conductivity ceramics can be brazed to various metals such asmay be used in the construction of header assembly 700. In contrast,second feedthru 812 in the illustrated embodiment comprises a materialthat is generally useful as a heat conductor, such as a metal. Variouscopper-tungsten alloys are examples of metals that are suitable in someapplications. Thus, platform 800 is generally configured to combine heatconductive elements with non-heat conductive elements so as to produce adesired effect or result concerning the device wherein platform 800 isemployed.

[0061] In connection with the foregoing, it should be noted further thatceramics and metals are exemplary materials only and any other materialor combination thereof that will facilitate implementation of thefunctionality disclosed herein may alternatively be employed. Moreover,other embodiments of the invention may employ different arrangements andnumbers of, for example, conductive and non-conductive feedthrus, orfeedthrus having other desirable characteristics. Accordingly, theillustrated embodiments are exemplary only and should not be construedto limit the scope of the invention in any way.

[0062] With respect to their configurations, the geometry of both firstfeedthru 802 and second feedthru 812 may generally be configured asnecessary to suit the requirements of a particular application ordevice. In the exemplary embodiment illustrated in FIGS. 4A through 4D,second feedthru 812 incorporates a step 812A feature which serves to,among other things, provide support for cooling device 900 and, asdiscussed in further detail below, to ensure that devices mounted tocooling device 900 are situated at a desirable location and orientation.As further indicated in FIG. 4D, for example, second feedthru 812defines a semi-cylindrical bottom that generally conforms to the shapeof cap 704 and contributes to the stability of cooling device 900, aswell as providing a relatively large conductive mass that aids in heatconduction to and/or from, as applicable, cooling device 900 and otherdevices.

[0063] As suggested earlier, platform 800 also serves to provide supportto cooling device 900. Directing renewed attention now to FIGS. 4Athrough 4D, details are provided concerning various aspects of coolingdevice 900. In particular, a cooling device 900 is provided that ismounted directly to platform 800. In an exemplary embodiment, coolingdevice 900 comprises a thermoelectric cooler (“TEC”) that relies for itsoperation and usefulness on the Peltier effect wherein electrical powersupplied to the TEC may, according to the requirements of a particularapplication, cause selected portions of the TEC to generate heat and/orprovide a cooling effect. Exemplary construction materials for the TECmay include, but are not limited to, bismuth-telluride combinations, orother materials with suitable thermoelectric properties.

[0064] Note that the TEC represents an exemplary configuration only, andvarious other types of cooling devices may alternatively be employed asrequired to suit the dictates of a particular application. By way ofexample, where active temperature control of one or more electronicdevices 1000, aspects of which are discussed in more detail below, isnot required, the TEC may be replaced with a thermally conductive spaceror similar device.

[0065] In addition to providing heating and/or cooling functionality,cooling device 900 also includes a submount 902 that supports variouselectronic devices 1000 such as, but not limited to, resistors,capacitors, and inductors, as well as optical devices such as mirrors,lasers, and optical receivers. Thus, cooling device 900 is directlythermally coupled to electronic devices 1000.

[0066] In one exemplary embodiment, the electronic devices 1000 includea laser 1002, such as a semiconductor laser, or other optical signalsource. With regard to devices such as laser 1002, at least, coolingdevice 900 is positioned and configured to ensure that laser 1002 ismaintained in a desired position and orientation. By way of example, insome embodiments of the invention, cooling device 900 is positioned sothat an emitting surface of laser 102 is positioned at, and alignedwith, a longitudinal axis A-A of header assembly 700 (FIG. 4C).

[0067] Note that although reference is made herein to the use of a laser1002 in conjunction with cooling device 900, it should be understoodthat embodiments employing laser 1002 are exemplary only and thatadditional or alternative devices may likewise be employed. Accordingly,the scope of the invention should not be construed to be limited solelyto lasers and laser applications.

[0068] In at least some of those embodiments where a laser 1002 isemployed, a photodiode 1004 and thermistor 1006 (see FIG. 4D) are alsomounted to, or proximate, submount 902 of cooling device 900. Ingeneral, photodiode 1004 is optically coupled with laser 1002 such thatphotodiode 1004 receives at least a portion of the light emitted bylaser 1002, and thereby aids in gathering light intensity dataconcerning laser 1002 emissions. Further, thermistor 1006 is thermallycoupled with laser 1002, thus permitting the gathering of dataconcerning the temperature of laser 1002.

[0069] In some embodiments, photodiode 1004 comprises a 45 degreemonitor photodiode. The use of this type of diode permits the relatedcomponents, such as laser 1002 and thermistor 1006 for example, to bemounted and wirebonded on the same surface. Typically, the 45 degreemonitor diode is arranged so that light emitted from the back of laser1002 is refracted on an inclined surface of the monitor diode andcaptured on a top sensitive surface of the monitor diode. In this way,the monitor diode is able to sense the intensity of the optical signalemitted by the laser.

[0070] Note that in those embodiments where a laser 1002 is employed,cap 704 includes an optically transparent portion, or window, 704Athrough which light signals generated by the laser 1002 are emitted.Similarly, in the event electronic device 1000 comprises other opticaldevices, such as an optical receiver, cap 704 would likewise include awindow 704A so as to permit reception, by the optical receiver, of lightsignals. As suggested by the foregoing, the construction andconfiguration of cap 704 may generally be selected as required to suitthe parameters of a particular application.

[0071] In view of the foregoing general discussion concerning variouselectronic devices 1000 that may be employed in conjunction with coolingdevice 900, further attention is directed now to certain aspects of therelation between such electronic devices 1000 and cooling device 900. Ingeneral, cooling device 900 may be employed to remove heat from, or addheat to, one or more of the electronic devices 1000, such as laser 1002,in order to achieve a desired effect. As discussed in further detailherein, the capability to add and remove heat, as necessary, from adevice such as laser 1002, may be employed to control the performance oflaser 1002.

[0072] In an exemplary embodiment, the heating and cooling, asapplicable, of electronic devices 1000 is achieved with a cooling device900 that comprises a TEC. Various aspects of the arrangement anddisposition of electronic devices 1000, as well as cooling device 900,serve to enhance these ends. By way of example, the fact that electronicdevices 1000 are mounted directly to cooling device 900 results in arelatively short thermal path between electronic devices 1000 andcooling device 900. Generally, such a relatively shorter thermal pathbetween components translates to a corresponding increase in theefficiency with which heat may be transferred between those components.Such a result is particularly useful where devices whose operation andperformance is highly sensitive to heat and temperature changes, such aslasers, are concerned. Moreover, a relatively short thermal path alsopermits the transfer of heat to be implemented relatively more quicklythan would otherwise be the case. Because heat transfer is implementedrelatively quickly, this exemplary arrangement can be used toeffectively and reliably maintain the temperature of laser 1002 or otherdevices.

[0073] Another aspect of at least some embodiments relates to thelocation of cooling device 900 relative, not just to electronic devices1000, but to other components, devices, and structures of headerassembly 700. In particular, because cooling device 900 is located sothat the potential for heat transmission, whether radiative, conductive,or convective, from other components, devices, and structures of headerassembly 700 to cooling device 900 is relatively limited, the passiveheat load imposed on cooling device 900 by such other components andstructures is relatively small. Note that, as contemplated herein, the“passive” heat load generally refers to heat transferred to coolingdevice 900 by structures and devices other than those upon which coolingdevice 900 is primarily intended to exert a heating and/or coolingeffect. Thus, in this exemplary embodiment, “passive” heat loads refersto all heat loads imposed on cooling device 900 except for those heatloads imposed by electronic devices 1000.

[0074] The relative reduction in heat load experienced by cooling device900 as a consequence of its location has a variety of implications. Forexample, the reduced heat load means that a relatively smaller coolingdevice 900 may be employed than would otherwise be the case. This is adesirable result, particularly in applications such as header assemblieswhere space may be limited. As another example, a relatively smallercooling device 900, at least where cooling device 900 comprises a TEC,translates to a relative decrease in the amount of electrical powerrequired to operate cooling device 900.

[0075] Another consideration relating to the location of cooling device900 concerns the performance of laser 1002 and the other electroniccomponents 1000 disposed in hermetic chamber 706. In particular, theplacement of cooling devices 900, such as TECs that include a “cold”connection, in hermetic chamber 706 substantially forecloses theoccurrence of condensation, and the resulting damage to other componentsand devices of header assembly 700, caused by the cold connection, thatmight otherwise result if cooling device 900 were located outsidehermetic chamber 706.

[0076] In addition to the heat transfer effects that may be achieved byway of the location of cooling device 900, and the relatively shortthermal path that is defined between cooling device 900 and theelectronic devices 1000 mounted to submount 902 of cooling device 900,yet other heat transfer effects may be realized by way of variousmodifications to the geometry of cooling device 900. In connection withthe foregoing, it is generally the case that by increasing the size ofcooling device 900, a relative increase in the capacity of coolingdevice 900 to process heat will be realized.

[0077] In this regard, it should be noted that it is the case in manyapplications that the diameter of base 702 is often constrained to fitwithin certain predetermined form factors or dimensional requirementsand that such form factors and dimensional requirements, accordingly,have certain implications with respect to the geometric and dimensionalconfiguration of cooling device 900.

[0078] By way of example, the diametric requirements placed on base 702may serve to limit the overall height and width of cooling device 900(see, e.g., FIG. 4D). In contrast however, the overall length of headerassembly 700 is generally not so rigidly constrained. Accordingly,certain aspects of cooling device 900, such as its length for example,may desirably be adjusted to suit the requirements of a particularapplication. In the case of a TEC, for example, such a dimensionalincrease translates into a relative increase in the amount of heat thatcooling device 900 can process. As noted earlier, such heat processingmay include transmitting heat to, and/or removing heat from, one or moreof the electronic components 1000, such as laser 1002.

[0079] Moreover, various dimensions and geometric aspects of coolingdevice 900 may be varied to achieve other thermal effects as well. Byway of example, in the event cooling device 900 comprises a TEC, arelatively smaller cooling device 900 will permit relatively quickerchanges in the temperature of electronic devices 1000 mounted thereto.In the case where electronic device 1000 comprises a laser, thiscapability is particularly desirable as it lends itself to control oflaser performance through the vehicle of temperature adjustments.

[0080] Turning now to consideration of the power requirements forcooling device 900, at least where it comprises a TEC, and electronicdevices 1000, it was suggested earlier herein that those devicestypically rely for their operation on a supply of electrical power.Generally, the TEC must be electrically connected with platform 800 sothat power for the operation of the TEC, transmitted from a power source(not shown) to platform 800, can be directed to the TEC. Additionally,power is supplied to electronic devices 1000 by way of platform 800, andelectronic devices 1000 must, accordingly, be connected with one or moreof the conductive pathways 806 of platform 800.

[0081] The foregoing electrical connections and configurations may beimplemented in a variety of ways. Various aspects of exemplaryconnection schemes are illustrated in FIGS. 4A, 4B and 4E. Withreference first to FIG. 4B, the underside of submount 902 of coolingdevice 900 is connected with conductive elements 814 disposed on theunderside of first feedthru 802, by way of connectors 816 such as, butnot limited to, wire bonds. Such conductive elements 814 may beelectrically connected with selected conductive pathways 806 (see FIG.4A) and/or connector 810, that are ultimately connected with anelectrical power source (not shown).

[0082] Directing attention next to FIG. 4A, details are providedconcerning various aspects of the electrical connection of electronicdevices 1000 disposed on submount 902. As noted earlier, and illustratedin FIG. 4A, some embodiments of platform 800 include one or more cutouts811, or other geometric feature, that permits direct connection ofelectronic devices 1000, such as laser 1002 to one or more conductivepathways 806 disposed within first feedthru 802 of platform 800. Thisconnection may be implemented by way of connectors 818 such as bondwires, or other appropriate structures or devices. In addition to theaforementioned connection, and as illustrated in FIG. 4E, at least someembodiments of the invention further include a flex circuit 820, orsimilar device, which serves to electrically interconnect platform 800of header assembly 700 with another device, such as a printed circuitboard.

[0083] With attention now to FIGS. 4A through 4D, details are providedconcerning various operational aspects of header assembly 700. Ingeneral, power is provided to laser 1002 and/or other electricalcomponents 1000 by way of connector 810, conductive pathways 806, andconnectors 818. In response, laser 1002 emits an optical signal. Heatgenerated as a result of the operation of laser 1002, and/or otherelectronic components 1000, is continuously removed by cooling device900, which comprises a TEC in at least those cases where a laser 1002 isemployed in header assembly 700, and transferred to second feedthru 812upon which cooling device 900 is mounted. Ultimately, second feedthru812 transfers heat received from cooling device 900 out of headerassembly 700.

[0084] Because cooling device 900 is disposed within hermetic chamber706, the cold junction on cooling device 900, where it comprises a TEC,does not produce any undesirable condensation that could harm othercomponents or devices of header assembly 700. Moreover, the substantialelimination of passive heat loads on cooling device 900, coupled withthe definition of a relatively short thermal path between electroniccomponents 1000, such as laser 1002, and cooling device 900, furtherenhances the efficiency with which heat can be removed from suchelectronic components and, accordingly, permits the use of relativelysmaller cooling devices 900. And, as discussed earlier, the relativelysmall size of cooling device 900 translates to a relative decrease inthe power required to operate cooling device 900. Yet other operationalaspects of embodiments of the invention are considered in further detailbelow in the context of the discussion of a laser control system.

[0085] While, as noted earlier in connection with the discussion ofFIGS. 4A through 4D, certain effects may be achieved by locating coolingdevice 900 within hermetic chamber 706, it is nevertheless desirable insome cases to locate the cooling device outside of the hermetic chamber.Aspects of an exemplary embodiment of such a configuration areillustrated in FIG. 4F, where an alternative embodiment of a headerassembly is indicated generally at 1100. As the embodiment of the headerassembly illustrated in FIG. 4F is similar in many regards to one ormore of the embodiments of the header assembly discussed elsewhereherein, the discussion of FIG. 4F will focus primarily on certainselected aspects of the header assembly 1100 illustrated there.

[0086] Similar to other embodiments, header assembly 1100 includes abase 1102 having a device side 1102A and a connector side 1102B, throughwhich a platform 1200 passes in a substantially perpendicularorientation. The platform 1200 includes an inside portion 1202A and anoutside portion 1202B. One or more electronic devices 1300 are attachedto inside portion 1202A of platform 1200 so as to be substantiallyenclosed within a hermetic chamber 1104 defined by a cap 1106 and base1102. In the event that electronic device 1300 comprises an opticaldevice, such as a laser, cap 1106 may further comprise an opticallytransparent portion, or window, 1106A to permit optical signals to betransmitted from and/or received by one or more electronic devices 1300disposed within hermetic chamber 1104.

[0087] With continuing reference to FIG. 4F, platform 1200 furthercomprises a first feedthru 1204, upon which electronic devices 1300 aremounted, joined to a second feedthru 1206 that includes an insideportion 1206A and an outside portion 1206B. The outside portion 1206B ofsecond feedthru 1206 is, in turn, thermally coupled with a coolingdevice 1400. In the illustrated embodiment, cooling device 1400comprises a TEC. However, other types of cooling devices mayalternatively be employed.

[0088] In operation, heat generated by electronic devices 1300 istransferred, generally by conduction, to second feedthru 1206. The heatis then removed from feedthru 1206 by way of cooling device 1400 which,in some embodiments, comprises a TEC. As in the case of otherembodiments, a TEC may also be employed, if desired, to add heat toelectronic devices 1300.

[0089] Thus positioned and arranged, cooling device 1400 is able notonly to implement various thermal effects, such as heat removal or heataddition, with respect to electronic devices 1300 located inside oroutside hermetic chamber 1104, but also operates to process passive heatloads, which may be conductive, convective and/or radiative in nature,imposed by various components such as the structural elements of headerassembly 1500. As noted herein in the context of the discussion ofvarious other embodiments, variables such as, but not limited to, thegeometry, placement, and construction materials of platform 1200 andcooling device 1400 may be adjusted as necessary to suit therequirements of a particular application.

[0090] As suggested earlier, at least some embodiments of the coolingdevice may be usefully employed in the context of a laser controlsystem. Directing attention now to FIG. 5, various aspects of anexemplary embodiment of a laser control system, indicated generally at2000, are illustrated.

[0091] As indicated in FIG. 5, laser control system 2000 includes atemperature sensing device 2002, such as a thermistor, which isthermally coupled with a laser 2004, such as a semiconductor laser.Laser control system 2000 further includes a light intensity sensingdevice 2006, such as a photodiode, that is optically coupled with laser2004. Further, a TEC. 2008 is thermally coupled with laser 2004. In atleast one embodiment, such thermal coupling is achieved by mountinglaser 2004 directly to a submount of TEC 2008. Laser control system 2000further includes a control circuit 2010 configured to receive inputsfrom temperature sensing device 2002 and light intensity sensing device2006, and to send corresponding control signals to a power source 2012in communication with TEC 2008.

[0092] In general, operation of laser control system 2000 proceeds ashereafter described. In particular, the intensity of the optical signalemitted by laser 2004 is sensed, either directly or indirectly, by lightintensity sensing device 2006. Light intensity sensing device 2006 thentransmits, from time to time, a corresponding signal to control circuit2010. In at least some embodiments, the temperature of laser 2004 may beregulated by TEC 2008 so as to achieve wavelength stabilization. Thiscan be achieved by way of control circuit 2010 and power source 2012.

[0093] Additionally, temperature sensing device 2002 is positioned andconfigured to measure the temperature of laser 2004 and transmit, fromtime to time, a corresponding signal to control circuit 2010. Based uponinputs received from temperature sensing device 2002 and light intensitysensing device 2006, control circuit 2010 is able to implement changesto the temperature of laser 2004 by way of power source 2012 and TEC2008.

[0094] In particular, because TEC 2008 may be configured to add and/orremove heat from laser 2004, laser control system 2000 thus affords theability to, among other things, change and/or maintain the temperatureof laser 2004 as desired or required by a particular application. Thuscontrol circuit 2010 cooperates with TEC 2008 to control both thedirection and amount of heat flow with respect to laser 2004. In thisway, various operational parameters of the signal emitted by laser 2004may desirably be adjusted.

[0095] That is, embodiments of laser control system 2000 are capable ofnot only maintaining the temperature of active devices such as laser2004 below a critical value at which laser 2004 performance begin todegrade and reliability becomes an issue, but embodiments of lasercontrol system 2000 also enable control of the temperature of activedevices such as laser 2004 at a given value independent of ambienttemperature conditions, so as to achieve certain ends such as, in thecase of laser 2004 operation for example, wavelength stabilization.

[0096] The present invention may be embodied in other specific formswithout departing from its spirit or essential characteristics. Thedescribed embodiments are to be considered in all respects only asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

What is claimed is:
 1. A header assembly, comprising: a base having a first side and a second side; and a platform attached to the base and disposed in a predetermined orientation with respect to the base, the platform having an inside portion proximate the first side of the base and an outside portion proximate the second side of the base, and the platform including at least one conductive pathway extending substantially through the platform.
 2. The header assembly as recited in claim 1, wherein the base substantially comprises a metallic material.
 3. The header assembly as recited in claim 1, wherein when the platform is disposed in the predetermined orientation, the platform is substantially perpendicular to the base.
 4. The header assembly as recited in claim 1, wherein the platform extends through at least one side of the base.
 5. The header assembly as recited in claim 1, wherein the platform substantially comprises a ceramic material.
 6. The header assembly as recited in claim 1, wherein the platform has a thermal conductivity that is different than a thermal conductivity of the base.
 7. The header assembly as recited in claim 1, wherein the platform has a thermal conductivity that is relatively higher than a thermal conductivity of the base.
 8. The header assembly as recited in claim 1, wherein the at least one conductive pathway of the platform extends through the base.
 9. The header assembly as recited in claim 1, wherein the platform substantially comprises one of: aluminum nitride; and, beryllia.
 10. The header assembly as recited in claim 1, wherein the platform defines a recessed pocket configured and arranged to at least partly receive an optical component.
 11. The header assembly as recited in claim 1, wherein the platform is hermetically sealed to the base.
 12. The header assembly as recited in claim 1, further comprising a device mounted to the platform on the first side of the base.
 13. The header assembly as recited in claim 12, wherein the device comprises at least one of: an optical transmitter; and, an optical receiver.
 14. The header assembly as recited in claim 1, wherein the platform further comprises a connector located on the outside portion of the platform and connected at least indirectly with the at least one conductive pathway.
 15. A header assembly, comprising: a base having a first side and a second side; a platform attached to the base and disposed in a predetermined orientation with respect to the base, the platform having an inside portion proximate the first side of the base and an outside portion proximate the second side of the base, and the platform including at least one conductive pathway extending substantially through the platform; and a thermal control element attached at least indirectly to the platform.
 16. The header assembly of claim 15, wherein the thermal control element is mounted at least indirectly to the inside portion of the platform.
 17. The header assembly of claim 15, wherein the thermal control element comprises a thermoelectric cooler.
 18. The header assembly as recited in claim 15, wherein the base substantially comprises a metallic material.
 19. The header assembly as recited in claim 15, wherein when the platform is disposed in the predetermined orientation, the platform is substantially perpendicular to the base.
 20. The header assembly as recited in claim 15, wherein the platform extends through at least one side of the base.
 21. The header assembly as recited in claim 15, wherein the platform substantially comprises a ceramic material.
 22. The header assembly as recited in claim 15, wherein the thermal control element is configured to facilitate transfer of heat to, and from, an optical device mounted at least indirectly to the platform.
 23. The header assembly as recited in claim 15, further comprising at least one optical device mounted at least indirectly to the platform, the optical device being arranged for thermal communication with the thermal control element.
 24. The header assembly as recited in claim 15, further comprising a cap that cooperates with the base to define a hermetic chamber substantially enclosing the inside portion of the platform and the thermal control element.
 25. The header assembly as recited in claim 15, wherein the platform further comprises a connector located on the outside portion of the platform and connected at least indirectly with the at least one conductive pathway.
 26. A header assembly, comprising: a base substantially having a device side and a connector side; a platform attached to the base and disposed in a predetermined orientation with respect to the base, the platform having an inside portion proximate the first side of the base and an outside portion proximate the second side of the base, and the platform including at least one conductive pathway extending substantially through the platform; a device mounted indirectly to the inside portion of the platform; and means for transferring heat, the means for transferring heat being thermally coupled with the device.
 27. The header assembly as recited in claim 26, wherein the means for transferring heat facilitates removal of heat from the device.
 28. The header assembly as recited in claim 26, wherein the means for transferring heat facilitates the transfer of heat to the device.
 29. The header assembly as recited in claim 26, wherein the means for transferring heat facilitates positioning of the device.
 30. The header assembly as recited in claim 26, wherein the means for transferring heat aids in controlling performance of the device.
 31. The header assembly as recited in claim 26, wherein the device comprises at least one of: an optical transmitter; and, an optical receiver.
 32. An optoelectronic device, comprising: a header assembly, comprising: a base having a first side and a second side; a platform attached to the base and disposed in a predetermined orientation with respect to the base, the platform having an inside portion proximate the first side of the base and an outside portion proximate the second side of the base, and the platform including at least one conductive pathway extending substantially through the platform; and at least one optical device at least indirectly attached to the platform; and a printed circuit board electrically connected with the header assembly.
 33. The optoelectronic device as recited in claim 32, wherein the electrical connection between the printed circuit board and the header assembly is implemented by way of a flex circuit in electrical communication with the printed circuit board and with the platform.
 34. The optoelectronic device as recited in claim 32, wherein the at least one optical device comprises at least one of: an optical transmitter; and, an optical receiver.
 35. The optoelectronic device as recited in claim 32, wherein the base of the header assembly substantially comprises a metallic material.
 36. The optoelectronic device as recited in claim 32, wherein when the platform of the header assembly is disposed in the predetermined orientation, the platform is substantially perpendicular to the base.
 37. The optoelectronic device as recited in claim 32, wherein the platform of the header assembly extends through at least one side of the base.
 38. The optoelectronic device as recited in claim 32, wherein the platform of the header assembly substantially comprises a ceramic material.
 39. The optoelectronic device as recited in claim 32, wherein the platform of the header assembly further comprises a connector located on the outside portion of the platform and connected at least indirectly with the at least one conductive pathway and with the printed circuit board.
 40. The optoelectronic device as recited in claim 32, wherein the platform has a thermal conductivity that is relatively higher than a thermal conductivity of the base.
 41. The optoelectronic device as recited in claim 32, wherein the at least one conductive pathway of the platform extends through the base.
 42. The optoelectronic device as recited in claim 32, further comprising a thermal control element attached at least indirectly to the platform and arranged for thermal communication with the at least one optical device. 